Vfd sizing

Good evening all

What are the sizing guides for vfd inverters doesn't seem clear cut

Been given a 3 phase pillar drill that has a 0.37kw motor 220-240 50hz should I buy a slightly bigger vfd say 0.5kw

Many thanks

I would go with the 0.5 kW one. De-rating electronics generally leads to improved reliability.

Russell

De-rating for reliability is perhaps a slightly old fashioned idea. Especially in the home workshop where things are rarely (deliberately?) driven flat out.

I'd say buy oversize and derate a bargain low end model as these are generally not engineered to run full power for long periods. As always its price / performance ratio. Why pay extra to be able to run flat out for hours on end when full oomph is needed for maybe 5 minutes a month. If there is an r in it.

Buy the right size if getting a good brand name vector drive unit. Vector drives do a good job of self derating by their very nature as they largely only deliver the power needed. Getting the right size means that all the factory default safety settings will be set correctly. No need to delve into the manual and parameter lists to set the maximum current et al.

At those sizes there is little objective difference in actual £ between a larger, inexpensive, version to be run derated and a decent brand vector unit of the right size.

Clive

Edited By Clive Foster on 14/07/2020 17:42:46

+1 for that.
Additionally I'm given to understand under-rating by too much can lead to poorer control of the sinusoidal waveform.

I bought a new 0.75KW motor and an 0.75KW Schneider VFD. They work perfectly together, I heeded some advise not to waste money on a larger VFD as the safety features built in were designed to be matched to a similar power motor.

The VFD is designed to drive a certain size motor, so buy that size motor VFD.

A motor will take pretty much the same current irrespective of its load, the power factor alters to produce the output power.

I was reading Oxford Welders web site, and they have some interesting comments on inverter electronics. I have to say that it fits with my experience and what I have heard, they will die sooner or later in any case. The electronics is seriously loaded, and buying a larger VFD doesn't alter the fact that the recitifiers and smoothing caps are really being driven hard. The conduction angle off the mains is only a few 10's of degrees, and really won't change in a larger VFD.

In the factory I worked in we had many hundreds of VFDs but the specification only called up a few sizes of drive. Many were over rated for the motors but they were sophisticated drives and all the motor parameters were part of the setup so the drive would be optimised for the motor to be controlled. Personally I would buy a decent branded drive to match the motor and I would expect it to be able to deliver full performance for a long working life. The component most likely to fail are the capacitors and companies that repair and refurbish drives will replace them as a matter of course, long before terminal failure they will suffer a reduction in performance.

Mike

Hi Everyone, the motor on most pillar drills is seldom pushed to it's full power, if you look in the instructions it should give some maximum drill sizes for different materials. So a 3.7kW inverter should be enough, but you should also check the full load current of the motor marked as FLC on the rating plate. I have in the past had to fit the next size up inverter because of the FLC not the kW.

Make sure you set the motor current in the parameters this will allow the inverter to protect the motor in the case of an overload. Setting a longer acceration ramp (say 10 seconds) will reduce starting strain on both the motor and the inverter.

Hope this helps Tom.

Oh dear, I hope Bob doesn't think I'm out to make his life miserable by questioning two posts on the same day?

I believe 'A motor will take pretty much the same current irrespective of its load, the power factor alters to produce the output power' is wrong.

• The current drawn by a motor does vary with load. More work = more amps.
• Power Factor doesn't alter to produce output power. As I understand it PF is a measure of distribution efficiency as distorted by inductive or capacitive loads. Inductive electric motors cause current to lag behind the volts, breaking the simple W=VI relationship on which Electricity Bills are based. Plus other side-effects.

I'm not clear on Conduction Angles either. Bob may be assuming a particular type of controller, perhaps SCR or Triac based? Are they common in welders?

My lathe's VFD charges a bank of big capacitors with high-voltage DC derived from rectifiers (about 170° conduction angle) . Then the electronics simulate 3-phase AC by chopping up the DC supply. I don't think limited conduction angles are an issue in this design.

I agree about buying VFDs sized to match the motor, rather than over egging. Electronic capacity for work is mainly determined the devices ability to get rid of heat. In that sense bigger heat-sinks on the next model up wouldn't be a bad thing. But the money's probably wasted. Even cheap VFD's are able to apply current limits and shutdown when they get too hot. And lathe motors are often pretty idle in amateur service. They aren't thrashed for long periods and get plenty of time to cool off while the operator gets ready for the next stage. It's a vacuum cleaner motor in the hands of a house-proud housewife that deserves our sympathy!

I tested my 1.5kW rated hobby lathe with a power meter and even brutal high-speed 6 mm deep cuts into grotty steel didn't consume more than 1.2kW. In normal use I've never managed to get the motor warm to touch. Others might be less happy with the same machine. It's a largish lathe for the sort of work I do and I rarely push it. A busy workshop frequently needing to remove a lot of metal in a hurry might well warm her up! However, I suggest not worth blowing lots of cash on an oversized VFD unless the machine works much harder than average. In which case, the lathe should be sized for industrial work too!

Dave

+1 for not over sizing the drive.
The sellers who advcate using oversized drives are just making more money from you. In the case of lower quality drives that may have undersized heatsinks and marginal semiconductors they are also reducing their exposure to warranty claims at your expense. As others have said if parameters, particuarly urrent limits are correctly set a larger drive will not protect the motor. Less obviously, the control will not be as good. Any drive will have a dynamic range from minimum load to maximum. Minimum does not change significantly with drive size but maximum does. Modern drives are digital with a fixed number of "steps" (resolution) between min and max. Typical is 10 bits giving 1024 steps. If you us a 1kW drive on a 500W motor a most it will get to half power - 512th step. Thus you have haff the control resolution. Most users will not notice this but why pay more for poorer performance?

Robert G8RPI.

Oh dear, on some points I disagree with Bob and SoD.

For an induction motor at no load the current is low and so is the power factor (around 0.1 to 0.3), ie, the motor looks pretty much like an inductor. As the load increases the current increases and the power factor improves to around 0.7 to 0.9 (still inductive) at full load.

With a VFD all the above is isolated from the mains input. The classic rectifier/capacitor circuit has a short conduction angle. Once the capacitors are charged the rectifier diodes only conduct when the mains input voltage is higher than the voltage on the capacitors. So current only flows for a short period near the peak voltage of the input. This is bad for several reasons. One, the rectifier diodes need to carry a much larger current (for a short period) to supply the nominal power rating of the device. That cause more heating and the need for bigger heatsinks. Two, current is taken from the mains as a series of short spikes, which have a high harmonic content. That upsets the energy suppliers. To comply with regulations a cheaper VFD (with a simple front end rectifier) should be fed from a filter so that the assembly looks like a resistive load from the mains.

Larger and/or better quality (more expensive ) VFDs may have a power factor corrector (PFC) at the front end. The PFC is basically a DC-DC boost converter, but with a control loop that forces the current draw to be proportional to the input voltage, ie, resistive.

Andrew

Gentlemen, wasn't the question I have a 3 phase pillar drill that has a 0.37kw motor 220-240 50hz should I buy a slightly bigger vfd say 0.5kw. I haven't notice a straight forward answer, could it be on the way any time soon,

YES! But doesn't need to be that much bigger so 0.5 kW should be fine.

The first reply seemed pretty straightforward to me?

Andrew

That may be because there is no straight answer. As an electrician taking on this job for a customer I would want to see the pillar drill and check for my self before fitting an inverter. To insure the correct size was fitted. Too small risks repeated tripping and even burning out the inverter; too big costs more than it needs to and far too big the inverter may not work correctly and may damage both the inverter and the motor.

Tom

Yes or no would answer the question but both answers benefit from some qualification. My querying mind is never satisfied by yes no or because I say so.

Mike

Is a 0.5kW VFD OK for a 0.37kW motor? Russell said yes, and no-one disagreed with him. 0.45kW would do too.

The discussion moving on more generally into the principles behind VFD sizing may have caused confusion, but may help others.

Rule of thumb: double the size is a bit much. In practice, I'd fit a spare 1.5kW VFD to a 350W motor to save a few quid. Not best practice though, and deliberately buying a new 1.5kW VFD for the same job would be daft.

I'm worrying about what Andrew said. Turns out a 22A dc load is an average 11A per diode, no problem, but a 20% ripple means a peak current of 164A. What makes my head hurt is whether it matters! If I buy a 20A Bridge Rectifier, its rated for 20A average, and - on average - the sums work out. Provided the rectifier's average rating isn't exceeded, it should be OK. And a bog-standard 1000piv 50A rectifier costing £3.37 has a surge rating of 400A, which is reassuring. Rectifiers do die, but not as often as the big amp number suggests.

What rectifier spikes do to the mains is another question. Can't help thinking of the good old days when the nation's millions watched television on valve sets drawing between 80W and 600W for big screen colour. Their PSUs ignored the negative and drank deep of the positive. I don't recall lack of positivity being a problem, but maybe the horror broke strong men at the power station.

Dave

Be careful - datasheet surge ratings are often listed under maximum ratings and can be non-repetitive. So you shouldn't be subjecting the diodes to them every cycle.

Andrew

Just don't go too OTT. if the VFD is way too big, its safety features might not work on a smaller motor. for ex if you have a 3HP VFD, the internal safety, overload protection might not perform correctly and you risk not protecting the motor.

also I suggest 1 VFD per 1 motor.

Greg

On top of all the arguments above (I agree with AJ, BTW) there may be a problem of how the motor has been rated and its efficiency. Some are ‘specced’ on electrical input and some are ‘rated’ on mechanical power out. They are not the same.

Most VFDs cope well with normal motor starting surges. Not all VFDs have good overload limit trips. There are often limits to how low one can set running current parameters (if those parameters are even available on the cheapest offerings).